It has long been known that when cost is no object, and the ultimate in electromagnetic shielding of air vents is necessary, that hexagonal metal honeycombs which are held together by soldering or welding are the material of choice. Because all abutting surfaces of the undulations of the hexagon that form the honeycomb pattern are held together by a conductive bond, the maximum electromagnetic shielding which can be derived from a body of honeycomb material is in fact attained.
Such material is very costly. Its cost can be justified for very critical installations, especially for military applications where cost is subordinate to reliability and capability. This is not the situation for most commercial (and also for many military) applications. For example, while an ultimate shielding of about 120 db at relatively high frequencies (1 GHz) can be attained with soldered or welded-together honeycomb structure there are many applications where only about 40 to 60 dB is necessary, and in which the cost of soldered or welded honeycomb for the panels cannot be justified.
This is not a new problem. Previous efforts have been made to utilize honeycomb which is constructed of undulations of metal foil which are joined at abutting segments by an epoxy resin. Such foils, without further treatment, do provide some shielding, perhaps as high as 60 db (at 1 GHz) but also as little as 30 db. This lack of reliability and consistency from panel to panel renders the use of such honeycomb filters unsuitable for many important applications where cost is a substantial criterion.
Honeycombs formed by joining adjacent undulations with an epoxy inherently conduct an electric current better in the direction of the undulations than across them. The undulations provide a continuous electrical circuit in their direction. However, the epoxy joinders between them are poor conductors, so the conductivity in the direction across them is poor.
Because the shielding property can be increased by conductively joining the undulations together, numerous ways conductively to join them have been suggested. One way is to pierce the abutting segments of the undulations with a probe (pin hole) that carries some of the metal from one undulation to its neighbor. This does provide some interconnection between the undulations, but its effect is rather small, and is not uniform, predictable or reliable.
Another means is by a relatively thick (0.0005 to 0.001 inches) electroless nickel-plating of the entire honeycomb structure. This can provide, while using an epoxy assembled honeycomb, performance nearly equal to that of a soldered or welded honeycomb. The problem is the cost. The surface area of the honeycomb is large, and therefore so is the cost of nickel-plating it. Due to the cost, one is as well off using the soldered or welded honeycomb.
Yet another suggested arrangement is to electroplate the honeycomb structure with a coating such as tin or cadmium. Again the cost is relatively high, although not as high as the cost of a honeycomb whose entire surface has been nickel plated or having the undulations soldered or welded together. This arrangement has been known to add additional shielding to the conventional honeycomb panel. However, it can also suffer from the acid treatment which is involved. It only improves the "bridge" created by contact points of metal that result from the sawing operation used in creating panels from honeycomb blocks. These do provide some conductive connection, but if they are etched off in the electroplating process, the shielding quality of the conventional panel can be significantly reduced (a loss of as much as 40 dB is not unusual), and the result is therefore not uniform, predictable or reliable.
It is an object of this invention to provide a cost effective honeycomb panel by performing mechanical processes on the face of the panel, and whose shielding property is reliable and reproducible.
Honeycomb panel wafers are formed from slices cut from large blocks of honeycomb by a sawing operation. The teeth of the saw tend to sever the panel, and while they do, some of them pull shard-like portions of one undulation across the epoxy gap, where they touch against the opposite undulation. This provides a few conductive bridges between the abutting undulations. In practice the undulations are aluminum alloy about 0.002 inches thick, and the epoxy bond is about 0.0005 inches thick. However, the presence, location, and number of these bridges is random and not reliable. Their formation is subject to variations of the sharpness and accuracy of the teeth, the speed of the saw cut and the pressure of the saw against the honeycomb. With so many variables, repetitive consistency is not to be expected. Still, improvement in shielding performance is noticeable compared to a panel which does not have these bridges.
It is an object of this invention to perform a mechanical finishing operation on the face of a panel to provide a substantially continuous and reliable metal bridge from one undulation to its neighbor, the bridge being made of self-material from one of the undulations, thereby "blending" the undulations together at their ends to make a continuous and reliable metallic bridge between them, all of this at a modest cost.
As a consequence of a straight-forward mechanical operation on the face of a conventional panel which before the operation inconsistently and unreliably provided a shielding between about 30-60 dB at 1 GHz, a consistent and reliable product is made which provides shielding of about 70 dB. This panel is good enough for a large preponderance of shielding installations.
Still more advantage can be attained by improving the contact of the bridges with the farther undulations, to a reliable 85 dB--a thirty times improvement over the basic improved panel.
With the use of a pair of these improved panels arranged as described herein, reliable shielding of about 110 dB can be provided. This is a surprising and unpredictable 10,000 times improvement in shielding obtained by a simple mechanical treatment of the two faces of the panel, and using two of them in face to face contact with each other.
Further, with a separation of the two improved panels, another doubling of the shielding to 116 dB can be attained.
It will be rare for every feature to be used in the same panel or pair of panels. The results attainable with a selected one or two of the features will suffice for nearly every shielding requirement.